For many sound-related applications, it is generally known and desired to apply noise suppression (or noise reduction) in audio or sound signals, e.g. comprising a speech signal part (when the user is actively speaking) and (e.g. occasional) ambient noise, in order to increase the sound quality, i.e. removing or minimising the ambient noise and obtaining the speech signal part (when present) as clearly as possible or as preferred for a given application. Noise suppression may both be applied when the user is speaking and when the user is not speaking.
Certain noise suppression methods e.g. involve the use of two or more receiving microphones or sound/acoustic transducers, sensors, transceivers, receivers, etc. (all simply referred to as a receiver in the following) and various schemes or algorithms to supress or remove noise.
In such schemes, certain known techniques normally require or assume that the noise is substantially similar at both receivers while the speech signal is more or less (but optimally only) present at one of the receivers.
Practically, this may e.g. be achieved by having a transmission difference between the two receivers for a speech signal and noise.
Achieving a transmission difference is normally done by having a relatively large physical distance between the two receivers as this provides different attenuation of and/or a time delay between the two signals (both signals including speech (when present) and noise (when present)) obtained at each receiver.
However, a drawback of such schemes is that the transmission difference of noise received by the two receivers in reality will not be identical and/or that the speech signals received by the two receivers in reality will be too similar to the transmission difference (between the receivers) of the noises, which may result in degraded noise suppression performance.
Noise suppression is of general interest in many audio-related applications including so-called normal use by regular users, e.g. using a headset for your phone or communications device, in traditional every-day noise environments.
In addition, noise suppression in comparatively severe noise environments presents its own challenges.
One use scenario is e.g. audio communication during transport of armed forces and/or during missions. As an example, the noise inside a helicopter or an armoured vehicle may be as much as 130 dB sound pressure level (SPL).
Another use scenario is e.g. audio communication in other (very) noisy and sometimes hazardous environments, e.g. as encountered by firefighters, emergency workers, police, and/or the like, where clear sound transmission and reception may even be crucial.
Certain noise suppression methods and systems also involve the use of one or more vibration pickups or transducers, e.g. such as one or more bone conduction microphones (BCMs) or corresponding. In such methods and systems, it is typically assumed and relied upon that the BCM, vibration pickup or transducer, etc. is perfectly shielded and therefore assumed practically speaking not being sensitive to ambient noise to any extent. However, contrarily that may not always or even often be the case which leads to noise contributions for a speech signal that are not addressed appropriately or even at all by many noise suppression methods and systems.
Patent specification US 2011/0135106 discloses a system for reducing ambient noise for mobile devices, such as a mobile phone, by using—in one embodiment—a combination of signals from an “in ear” speaker, a standard microphone, and a bone conduction microphone. According to this specification, the bone conduction microphone is assumed to be ideal in the sense that it is not sensitive to ambient noise to any extent and its signal is used accordingly. However, contrarily a bone conduction microphone may very well pick up or at least be influenced by airborne signals such as airborne speech and airborne noise causing vibrations to the bone conduction microphone interfering with or at least influencing registration or pick-up of “bone-conducting” vibrations propagating through the user. Not taking the presence of such airborne signal(s) into account may degrade the quality of the used ambient noise reduction scheme, especially in very noisy environments. The noise reduction scheme according to patent specification US 2011/0135106 uses adaptive filters and avoids adaptation during silence of the user and, at least in some embodiments, require calibration in a quiet environment.
Patent specification US 2014/0029762 relates to noise reduction in connection with a head-mounted sound capture device, e.g. glasses, comprising an air microphone and a vibration sensor where an equalizing transfer function between clear voice signals of the air microphone and the vibration sensor is e.g. determined during training or calibration with the user speaking in quiet environments where the ambient sound level is below a certain level for a certain period of time using the air microphone only.